A scanning microscope that scans an observation surface of a sample with an illumination light using a galvano-scanner, and acquires a two-dimensional observation image of the observation surface has been available. In such a scanning microscope, observation light generated from the observation surface by irradiation of illumination light is photo-electric converted and an electric signal acquired as a result is sampled, whereby data of each pixel of the observation image is generated.
If an electric signal is sampled at an equal time interval when the observation image is acquired, a scanning mirror of the galvano-scanner must be moved at a constant velocity during the period of acquiring the observation image, in order to acquire an undistorted observation image.
Normally a position (rotation angle) of the scanning mirror is in proportion to the voltage value of a driving signal of the galvano-scanner (hereafter called “driving voltage”), therefore if the scanning mirror is driven at a constant velocity, the driving voltage linearly changes along with time.
On the other hand, when the observation image is acquired, the observation surface is repeatedly scanned, hence after the scanning mirror is driven for a predetermined period at a constant velocity, driving for inverting the moving direction of the scanning mirror and returning the scanning mirror back to the original position is always required. During this time the scanning mirror is driven so as to decelerate, stop and reaccelerate, and the driving voltage of the driving signal for this driving changes non-linearly with respect to time. The observation image is not acquired in a period when the driving voltage changes non-linearly. Thus in the case of scanning by deflecting the scanning mirror, the driving signal has a period when the driving voltage linearly changes in the time direction and a period when the driving voltage non-linearly changes in the time direction.
If the scanning mirror is driven at high speed close to the limit of the performance of the scanner, it is known that the ratio of the resonance frequency components unique to the galvano-scanner in the frequency components of the driving signal become high, and operation of the scanning mirror becomes unstable.
In other words, the observation surface is normally scanned by a driving signal having a serrated waveform, where a period when the driving voltage changes linearly and a period when the driving voltage changes non-linearly are combined. In this case, high frequency components are included in the period when the driving voltage changes non-linearly, but as the driving velocity of the scanning mirror increases, the ratio of the resonance frequency components included in this non-linear period increases, which make the operation of the scanning mirror unstable.
Therefore in the case of driving the scanning mirror at high speed, the scanning mirror can be more stably driven if the scanning mirror is driven by a sinusoidal driving signal constituted by a single frequency component, rather than being driven by a serrated driving signal. However in the case of a sinusoidal driving signal, the change of amplitude of the driving signal becomes non-linear, because of the nature of a sinusoidal wave, which means that the rotation angle of the scanning mirror non-linearly changes with respect to time, and if an electric signal is sampled at a predetermined time interval to acquire an observation image, the observation image is distorted.
To solve this problem, a method of optically generating a sample clock using an optical member disposed on the rear face side of the scanning mirror is proposed, so that the sampling clock linearly changes with respect to the rotation angle of the scanning mirror, and an undistorted image is acquired (e.g. see Patent Document 1).
Another possibility is limiting an effective scanning range on a sample, and decreasing the influence of distortion of the observation image by using only a period having high linearity in the waveform of the driving signal. In this case, if the phase angle of a sinusoidal wave as a driving signal is 0 to 2π, the waveform of the driving signal changes almost linearly in the periods when the phase is near 0 and 2π, hence a less distorted observation image can be acquired even if sampling is performed at a predetermined time interval, if the scanning image is very small.
Another available method is generating a sampling clock with a variable interval using an electronic circuit, by sampling a position signal or a driving signal of a scanning mirror for a short time, and adjusting the oscillation frequency, which is a source of the sampling clock, so that the deviation of the position of the scanning mirror becomes constant (e.g. see Patent Document 2).    [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-147395    [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-15636
However in the case of the method for acquiring an observation image using only a period when the driving signal has high linearity, an area regarded as a straight line in a sinusoidal wave is so limited that the effective scanning range on the sample becomes small, and only an observation image of a narrow partial area of the sample can be acquired. In such a case, if the resolution of the observation image is increased, scanning time per pixel becomes short and quality of the observation image drops.
In the case of the method for generating a sampling clock by disposing an optical member on the rear face side of the scanning mirror, the optical member disposed on the rear face side of the scanning mirror and a detection circuit for generating the sampling clock are required, which makes configuration complicated and increases cost. In this method, a change of the amplitude of the scanning minor changes the moving range of the optical member, which means that a sampling clock in an arbitrary scanning range cannot be generated, and the scanning range is limited to a predetermined range.
In the case of variably changing the oscillation frequency to be a source of the sampling clock, configuration of the circuit to generate the clock for acquiring an image becomes complicated, and increases cost. Furthermore if the sampling clock is based on variable frequency, clock cycle becomes unstable, causing a drop in image quality of the observation image.